Inside Sin, known as just Sin in the menu (but called Inside Sin at the Monster Arena) is a location in Final Fantasy X, and the game's final dungeon. It is located inside Sin, a gargantuan whale-like beast that threatens all life on Spira. This is the only storyline location that must be manually filled in by the player by traversing the area, highlighting its unknown nature.

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The final area of inside Sin, Dream's End, is briefly glimpsed when Tidus is first absorbed into Sin from Dream Zanarkand. He is teleported to Dream's End, where a vision of Jecht stands on the central platform. After Tidus "swims" toward the platform he is transported to the submerged ruins. Another section, the Nucleus, is seen in the aftermath of the disastrous Operation Mi'ihen, where the spectral visions of the Crusaders are seen wandering around the more corporeal form of Bahamut'sfayth.

When they penetrate Sin's armor, the party makes their way inside Sin and toward the center, where Yu Yevon rests. On their way deeper into the beast the party faces Seymour Guado, who has been absorbed by Sin, for a final battle after which Yuna sends him at last. At the center of the gargantuan beast, in Dream's End, the party meets Jecht who transforms into Braska's Final Aeon. After being vanquished, Yu Yevon's spirit seeks a new host and possesses Yuna's aeons one by one. The party defeats the aeons until Yu Yevon has no more host bodies to possess and then defeats Yu Yevon himself. Upon Yu Yevon's death he stops summoning Sin and it explodes above Bevelle, while the party escapes on their airship.

Most of the areas are based on the ancient Zanarkand except that it is a distraught projection of it.

Sea of Sorrow(悲しみの海, Kanashimi no Umi?) - A haunting, red-tinged sea winding through a cloudless vista, it is traversed through a series of waterfalls and watering blue platforms. There are symbols of Yevon surrounding this place.

Garden of Pain(なげきの園, Nageki no Sono?) - A series of steps leading to a platform, it is where the battle with Seymour Omnis takes place. Once he has been defeated, this area is bypassed on all subsequent trips through Sin, but the staves will remain.

City of Dying Dreams.

City of Dying Dreams(死せる夢の部, Shiseru Yume no Bu?) - A maze of moving platforms and teleport glyphs which must be navigated. Various side paths lead to treasure chests containing weapons and items.

Tower of The Dead(死者の塔, Shisha no Tou?) - As the player approaches what appears to be an exit to the city, a large metal tower descends and blocks their path. Examining a glyph on the side of the tower will teleport them past the point of no return and to the next area.

The Nucleus(悪夢の中心, Akumu no Chuushin?, lit. Core of Nightmares) - A surreal environment of crystalline floors and walls; the goal is to touch the colored crystal orbs which appear in the area before they disappear. The camera angle is always moving, adding to the disorientation. An item is awarded each time a crystal is touched, but touching the icicles which sprout from the floor will cause a battle to begin. After touching ten crystals the player is transported to the next area. The area is briefly seen after Operation Mi'ihen's failure.

Dream's End.

Dream's End - A ruined version of Zanarkand's blitzball stadium with a platform leading to the center of the arena. It appears the pyreflies in the surrounding area reflect Jecht's thoughts and thus Sin's core looks like a warped version of Dream Zanarkand with a flaming Zanarkand Abes logo on the background. Here the party meets Jecht and fights Braska's Final Aeon. When it has been defeated, Yu Yevon reveals himself, searching for a new aeon to possess.

In the City of Dying Dreams area, the player will find a glyph on the right wall. The first glyph tells the player to kill ten individual fiends. After doing so, return to the glyph and touch it to reveal another. The player must kill ten more to open the next door and then 15 to open the final one. A quick way to achieve this is to battle a party of three enemies (like Exoray's group). After the player has opened all three doors, they will find a chest with a Lv. 4 Key Sphere.

In The Nucleus area, small floating egg-like crystals appear alongside spike crystals popping out of the ground. The player must touch the floating crystals and avoid the spike crystals. The egg crystals disappear after a short time, so grab them quickly when they appear. Each one will give an item in a random order:

The Dream's End (夢の終わり, Yume no Owari?) area is the representative arena of Final Fantasy X. Unlike most arenas, Dream's End is only used for the two fights against Jecht: one in Tidus's storyline and the other in Shade Impulse. No other bosses are fought at Dream's End, though a handful of cutscenes occur here, including Firion and the Onion Knight's respective conversations with Cosmos.

The arena consists of the ruined Zanarkand blitzball stadium surrounding a circular platform over a field of lava, which is treated as a banish trap. A massive replica of Jecht's sword appears on the central platform, much like after defeating Braska's Final Aeon in Final Fantasy X. Long grind bars run along the outer edge of the arena, connecting the central platform to the ruined "stands" on the opposite side. In the Ω version of the stage, every time a character guards, dodges, or lands an attack, the Bravery pool increases, and when damage is dealt it increases by an amount equal to the damage dealt.

The Moonflow is the battlegen item obtained through stage destruction in this area.